Monostable multivibrator circuit



March 5, 1957 J. D. SABLE 2,784,309

MoNosTABLE MULTIVIBRATOR CIRCUIT Filed Feb. 4, 1953 I l I l l I l Icz/Mf grief arr/fai Z/ f/f" \,.300 v INVENTOR. TERUMED- Salar-r By /wwn/ATTORNEY MONOSTBLE MULTIVIBRATOR CIRCUIT Jerome D. Sable, Stratford, N.J.,assignor to Radio Corporation of America, a corporation of DelawareApplication February 4, 1953, Serial No. 335,098

6 Claims. (Cl. Z50-27) This invention relates to monostablemultivibrator circuits, and more particularly to such circuits includinga cathode follower in a cross-coupling circuit to provide a very shortrecovery time.

Multivibrators normally comprise two vacuum tube electrode structureswith their input and output electrodes cross coupled so that when onetube conducts the other is cut 01T, and vice versa. In a monostablemultivibrator, there is a stable oit condition with the rst tubeconducting and the second tube cut oi (i. e. with negligible or nooutput current flowing in the tube). When the multivibrator receives atrigger pulse of desired sense and magnitude, the multivibrator circuitshifts to an unstable on condition with the first tube cut olf and thesecond tube conducting. The on condition obtains for a period of timedetermined by the time constant of the cross-coupling between the tubesand then the circuit automatically shifts back (returns) to the stableoff condition.

Multivibrators are commonly used in radar systems to provide a gating orshaped waveform starting atthe time of the transmitted radio-frequencypulse and ending prior to the time of the following transmitted pulse.It is often desirable to generate a gating waveform wherein the on timeis almost equal to the time between transmitted pulses. In this case,the multivibrator must have a very short recovery time, that is, afterswitching itself oi it must immediately become ready to be switched onagain by a trigger pulse. The desirability of having a multivibratorwith avery short recovery time is particularly great in systems of thetype having a modulated pulse repetition rate. This is so because theshorter the recovery time, the greater the distance range of the radarsystem with a given minimum period between transmitted pulses, and,unless the recovery time of a multivibrator is short, the on time maynot remain constant when triggered by a pulse train wherein the timebetween successive pulses varies. It is therefore an object of thisinvention to provide a multivibrator having a low possible ratio of olftime to on time, in the order of live percent or less.

lt is another object to provide an improved multivibrator having a lowimpedance output point so that the utilization circuits do not alter theoperation of the multivibrator. A

It is a further object to provide an improved multivibrator having anoutput wave which swings both positive and negative and which can bedirectly connected to the grid of a utilization tube without using acoupling capacitor or attenuation network,

It is a still further object to provide a multivibrator wherein theoutput electrode of one tube is directly connected to the inputelectrode of the other tube, without the use of a coupling capacitor, sothat faster response may be obtained.

,In accordance with the teachings ofthis invention, there is provided amonostable multivibrator circuit having a rst normally-conducting tubeelectrode structure ited States Patent y 2,784,309 Eiatented Mar. 5,1957 the grid side of the timing capacitor discharges sutii# ciently sothat the voltage on the grid of the rst multi-` vibrator tube exceedsthe grid cut-off voltage, at which time the lirst multivibrator tubebecomes conductive and the second multivibrator tube is cut olf.

A feature of the invention is the arrangement whereby the anode of thesecond multivibrator'tube is coupled through a cathode follower and atiming capacitor to the grid of the lirst multivibrator tube. Thecapacitor is part of an RC circuit which determines the on time of themultivibrator. After the multivibrator switches itself off (i. e.restores itself to the stable condition), the cathode follower rapidlyrecharges the timing lcapacitor to provide a very short recovery timefor the multivibrator. The cathode follower also provides a lowimpedance output point in the multivibrator.

Another feature of the invention is the use of values of direct currentsupply voltages on the multivibrator tubes that permit the plate of thelirst multivibrator tube to be directly connected to the grid of thesecond multivibraor tube. The absence of coupling elements results infaster response by the second multivibrator tube.

These and other objects, advantages and features of the invention willbe apparent to those skilled in the art from the following descriptiontaken together with the appended drawings, wherein:

Fig. l is a circuit diagram of a presently preferred form ot' theinvention; and

Fig. 2 is a chart of voltage waveforms at various designated points inthe circuit'of Fig. 1.

In Fig. 1, various leads are shown going to a direct current voltagesource (not shown) which in the present example provides potentials of300, 150, +150 and +300 volts relative to ground. Voltage sources arewell known in the art and in describing Fig. l, an element will bemerely saidto be connected to a certain voltage. A trigger inputterminal 9 is connected through coupling capacitor 10 to the grid 13 ofbuffer vacuum tube or electron discharge device 15. A grid bias resistor11 is connected from grid 13 to ground, and another grid bias resistor12 is connected from grid 13 to -150 volts. Resistors 11. and 12constitute a voltage divider which serves to bias buffer tube 15 belowcut-off, that is, to the condition of no current flow through the tube.Bulfer tube 15 includes a cathode 16 connected to ground and an anode 17connected to grid 19 of cathode follower vacuum tube o1' electrondischarge device 20. Tube 20 includes an anode 21 connected to +300volts anda cathode 22 connected through cathode resistor 23 to -150volts. Cathode 22 is also connected through an RC circuit consisting ofcapacitor 24 and resistor '24 to the grid 26 of a lirst vacuum tube orelect-ion discharge device 25 of a multivibrator including tubes 25 and30. One end of resistor 24 is connected to 150 volts. Multivibrator tube25 includes a cathode 27 connected to 300 volts and an anode 28connected through anode resistor 29 to -150 volts. Anode 28 is alsoconnected across grid resistor 35 to the grid 31 of the second vacuumtube or electron discharge device 30 of the multivibrator. Tube30'includes a cat-bode' 32 connected-'to 150 volts and an anode 33connected through' a plate 3 resistor 34 to +150 volts. Anode 33 is alsoconnected to grid 19 of cathode follower tube 20 and plate 17 of buffertube 15.

In the absence of a trigger input pulse of desired sense and magnitude,buffer tube is cut off by the grid bias provided by voltage divider gridresistors 11 and 12 connected between -150 volts and ground. Firstmultivibrator tube 25 is-conducting heavily because, while the grid 26is connected through resistor 24 to -150 volts, the cathode 27 isconnected directly to a more negative voltage of 300 volts, as a resultof which the grid is positive relative to its cathode. Secondmultivibrator tube 30 is cut olf because its cathode 32 is connected to--150 volts and its grid is connected to a more negative voltage on theanode 28 of first multivibrator tube 25. The anode 28 ofiirstmultivibrator tube 25 isxmore negative than -150 volts because of thevoltage drop ,in anode resistor 29 due to current drawn'therethrough bythe conducting tube 2S. Cathode vfollower tube is conducting because itsgrid 19 is connected through resistor 34 to +150 volts and its cathode22 is connected through cathode resistor 23 to -150 volts.

The operation of the circuit when a positive pulse is applied to inputterminal 9 will now be described. Reference will be made to the voltagewaveforms shown in Fig. 2 wherein the designations (a) through (e) areused to identify the waveforms at correspondingly designated points inthe circuit of Fig. 1. A positive pulse applied from input terminal 9through coupling capacitor 10 to grid 13 of buffer tube 15 renders thetube conductive. The current drawn through anode resistor 34 causes thevoltage at (b) to fall as shown in curve (b) of Fig. 2. This voltageapplied to the grid 19 of cathode follower 20 causes the voltage oncathode 22 to fall also because of the reduction in current throughcathode resistor 23. The voltage drop on cathode 22 is coupled through`capacitor 24 to the grid 26 of first multivibrator tube 25, as shown inwave (d) of Fig. 2, as a result of which the flow of current throughtube 2S ceases. When current through anode resistor 29 ceases, thevoltage on anode 2% rises to about -150 volts shown in wave (e) of Fig.2. This increased Voltage is applied to grid 31 of the secondmultivibrator tube 30 causing tube 30 to conduct heavily, and draw anodecurrent through anode resistor 34, and grid current through resistor 3S.The potential at (b) remains at the low value due .to current flowingthrough resistor 34 and tube 30, while buffer tube 15 returns to thecut-off condition at the end of the trigger pulse applied to inputterminal 9. As long as first multivibrator tube remains cut off, the-150 volts on its anode 23 which is applied to grid 31 of secondmultivibrator tube 30 maintains tube 30 in a heavily conductingcondition.

The negative square voltage wave applied from the cathode 22 of cathodefollower 20 through capacitor 24 to grid 26 of first multivibrator tube25, in the first instant appears entirely across resistor 24', anddrives grid 26 far below cut-off. Ffhe voltage on grid 26 immediatelystarts increasing exponentially as negative charge on the grid side ofcapacitor 24 discharges through resistor 24. After a period determinedby the time constant of capacitor 24 and resistor' 24', the voltage ongrid 26 reaches and exceeds the cut-off value and tube 25 startsconducting. The voltage on anode 28 of tube 25 and grid 31 of tube 30then starts to fall, causing the voltage of anode 33 of tube 30, grid 19and cathode 22 of cathode follower 20 to rise. This rise in voltage iscoupled by capacitor 24 to grid 26 of tube 25 causing a further increasein anode current drawn by the tube.

The action continues in a rapid regenerative manner until firstmultivibrator tube 25 is conducting heavily and second multivibratortube is cut olf. The initial conditions then obtain and the cycle isrepeated when another positive ,trigger pulse is applied to terminal 9.

A multivibrator constructed according to known systems of the prior artwould have the anode 33 of second multivibrator tube 30 coupled throughcapacitor 24 to the grid 26 of rst multivibrator tube 25. In the circuitof the invention illustrated in Fig. l, the feedback coupling betweenthe anode 33 and grid 26 includes a cathode follower 20 and thecapacitor 24. This arrangement permits of a very short recovery time inthe transition from the on condition to the off condition. The re coverytime depends on the speed with which capacitor 24 can be recharged afteriirst multivibrator tube 2S starts conducting. The Vcharging currentreaches capacitor 24 through cathode follower tube 20 which has a verylow impedance, and consequently the charging time constant is low. lnconventional multivibrators, the charging current reaches the capacitorthrough the anode resistor 34 of the second multivibrator tube 30. Sincean anode resistor has a relatively high impedance, consequently thecharging time constant is relatively long.

The multivibrator of Fig, l is also characterized by a very rapidtransition from the olf condition to the on condition. This rapidtransition results from the construction whereby the direct currentsupply voltages to the multivibrator tubes have such values as to allowthe use of a direct connection feedback coupling between the anode 2S offirst multivibrator tube 25 and the grid 31 of second multivibrator tube30 and the absence of coupling elements in this direct connection.

The following values are given by way of example, for a .cir-cuitconstructed according to Fig. 1.

Resistor 11 ohms 100,000 Resistor 12 do 470,000 Resistor 23 do 33,000Resistor 24' do- 1,000,000 Resistor 29 do 10,000 Resistor 35 do 100,000Resistor 34 do 15,000 Capacitor 10 c micromicrofarads 470 Capacitor 24do 1,000

The circuit provided waveforms as shown in Fig. 2

when operated with a frequency modulated trigger input Wave having arepetition rate of about 1400 pulses per second.

What is claimed is:

1. A monostable multivibrator comprising, a source of uni-directionalpotential including first, second, third, fourth and fifth terminals, inorder, from relatively most positive potential to relatively mostnegative potential; first, second, third, fourth and fifth resistors; afirst normally conductive electron discharge device including a cathodeconnected to said fth terminal, a grid connected thru said firstresistor to said fourth terminal, and a plate connected thru said secondresistor to said fourth terminal, and also thru said third resistor tosaid third terminal; a second normally non-conducting electron dischargedevice having `a cathode connected to said fourth terminal, a griddirectly connected to the plate of said first device, and a plateconnectedthru said fourth resistor to said second terminal; a cathodefollower device including a plate connected to said first terminal, agrid counected to the plate of said second device, and a cathodeconnected thru said fifth resistor to said fourth terminal; and a timingcapacitor coupling the cathode of said cathode follower device .with thegrid of said iirst device; whereby said capacitor and said tirstresistor constitute a timing circuit which determines the length of timesaid first device remains non-conductive.

2. A monostable multivibrator circuit having a stable state and anunstable state, comprising first and second electron discharge electrodestructures each having input and output electrodes, and means toregeneratively cross-couple the input and output electrodes of saidstructures so that when one `is .conductive the `other is cut-off, andvice versa, said .cross-coupling means including a connection capable ofvpassing direct current extending from the output electrode of saidfirst structure to the input electrode of said second structure, acathode follower in said regenerative cross-coupling means including anelectron discharge device having grid and cathode electrodes and acathode resistor connected to said cathode, a connection from said gridto the output electrode of said second structure, and a timingcapacitorresistor combination connected between said cathode and theinput electrode of said first structure, said capacitor-resistorcombination having such values to determine the time the multivibratorremains in its unstable state.

3. A multivibrator circuit comprising a rst and a second dischargedevice, each having a grid and a plate, means to cross-couple the gridsand anodes of said devices so that when said first device is conductingsaid second device is cut-off and when said first device is cut-off saidsecond device is conducting, said cross-coupling means including: adirect connection devoid of concentrated impedance from the anode ofsaid first device to the grid of said second device, a cathode followerincluding a discharge device having a grid, a cathode and a cathoderesistor connected to said cathode, a connection from the anode of saidsecond device to the grid of said cathode follower, a timing capacitorconnected between the cathode of said cathode follower and the grid ofsaid rst device, and a resistor connected from the grid of said firstdevice to a source of bias potential, said timing capacitor having suchvalue that it aids in determining the time during which said first tubeis made to be non-conductive.

4. A multivibrator circuit comprising first and second electrondischarge devices having cathode, grid and anode electrodes;regenerative cross-coupling means between the anodes and grids of saidfirst and second devices making one device conductive when the other iscut-off, and vice versa, said cross-coupling means including a directconnection devoid of concentrated irnpedlance between the anode of saidfirst device and the grid of said second device; a cathode followerincluding an electron discharge device having cathode, grid and anodeelectrodes and a cathode resistor connected to said cathode; aconnection from the grid of said cathode follower to the anode of saidsecond device; a resistance-capacitance timing circuit connected betweenthe cathode of said cathode follower and the grid of said first device;and a buffer circuit having an output terminal connected to the grid ofsaid cathode follower and having an input terminal receptive to anexternal triggering pulse.

5. A monostable multivibrator having alternate conductive states,comprising a Afirst normally conducting tube having grid and anodeelectrodes, a second normally non-conducting tube having grid and anodeelec trodes, regenerative cross-coupling means between the sanodes andgrids of said first and second tubes making one tube conductive when theother is cut-off, and vice versa, said cross-coupling means including adirect-connection feedback from the anode of said first tube to the gridof said second tube, a cathode follower tube in said regenerativecross-coupling means having grid and cathode electrodes, a connectionfrom said grid of the cathode follower tube to the anode of said secondtube, and a timing capacitor-resistor combination connected between saidcathode of the cathode follower and the grid of said first tube, saidcapacitor-resistor combination having such values as to determine theduration of each of the alternate conductive states.

6. A monostable multivibnator comprising, first and second electrondischarge devices which are alternately conductive, said devices eachhaving input and output electrodes; first regenerative cross-couplingmeans couF pling an output electrode of said first device to an inputelectrode of said second device, said first coupling means constitutinga direct connection devoid of concentrated impedance; secondregenerative cross-coupling means coupling an output electrode of saidsecond device to an input electrode of said first device, said secondcoupling means including a cathode follower electron discharge devicehaving grid and cathode electrodes, a direct metallic connection fromsaid grid to an output electrode of said second device, and aresistance-capacitance timing circuit coupled between said cathode andan input electrode of said first device, said timing circuit havingvalues determining the duration of alternate states of conduction ofsaid first and second devices; and means to bias said three devices sothat said first and second coupling means make the first deviceconductive when the second is cut-off, and vice versa.

References Cited in the file of this patent UNITED STATES PATENTS2,155,210 Young Apr. 18, 1939 2,442,769 Kenyon June 8, 1948 2,469,031Canfora May 3, 1949 2,515,271 Smith July 18, 1950 2,642,532 MofensonJune 16, 1953 FOREIGN PATENTS 587,940 Great Britain Apr. 18, 1939

